1. Field of the Invention
This invention relates to a hydraulic control system for an automatic transmission, especially to such a system for pressurizing a clutch rapidly during low temperature or high load conditions, and at a controlled lower rate at normal temperature and normal load conditions.
2. Description of the Prior Art
The period during which a gearshift is made by an automatic transmission from any forward gear to reverse, or from reverse to a forward gear must extend no longer than five to ten seconds under any condition and should be shorter than that at ambient temperature above 0.degree. F. Such transmissions prevent sudden, harsh gear engagements by controlling the time rate at which pressure rises in a clutch or brake required to complete the gearshift. However, when power demand is high and at other conditions producing high throttle valve pressure, the clutch or brake must engage rapidly to meet operator expectations. When temperature is low and viscosity of transmission fluid is high, engagements are inherently sluggish because of delay in filling the clutch cylinder due to flow resistance.
To produce smooth rapid gearshifts at all temperatures, and rapid clutch engagements at high throttle valve pressure conditions, the hydraulic control circuit supplies fluid to the clutch through multiple control valves arranged in parallel and through a shuttle valve where fluid from both control valves combines to produce a single fluid source connected to the cylinder of the clutch. One pressure source supplies the control valve of an accumulator used during forward upshifts and low-power reverse engagement at normal ambient temperature. To control the time rate at which pressure increases within the clutch at normal throttle valve pressure, the clutch is supplied exclusively from the output of the control valve associated with the accumulator.
However, at high power conditions, e.g., when the accelerator pedal of the vehicle is depressed substantially, the throttle valve is opened a substantial amount, engine speed is high, vehicle speed is high, or ambient temperature is low, then a throttle valve produces high pressure output, which causes a second source of hydraulic pressure, not associated with accumulator operation, to be connected to the shuttle valve and clutch. When throttle valve pressure is low, the clutch is supplied from the hydraulic circuit that includes the accumulator and its associated control valve. But, when throttle valve pressure is high, the control senses need for rapid engagement of the clutch and connects an auxiliary fluid pressure source to the shuttle valve to avoid delay in engagement.
The accumulator control valve regulates supply pressure by balancing a control feedback pressure force against a spring force, vents regulated output pressure when feedback pressure is high, and closes the opening to vent when feedback pressure is again low. However, when ambient temperature is cold, viscosity of hydraulic fluid within the shuttle valve is high. This delays, and may prevent entirely, closure of a shuttle valve port connected to the accumulator control valve. This port remains open while the control valve is regulating. The clutch cylinder and shuttle valve are successively opened and closed to vent while the accumulator control valve regulates output pressure.
In the early stage of clutch engagement, clearances among friction discs within the clutch are taken up as the clutch cylinder is filled at low pressure. During this period, while the shuttle valve is open to the control valve and the control valve is regulating, instead of the clutch cylinder filling with hydraulic fluid to take up the clearances between the components of the clutch, hydraulic fluid is directed back to the control valve vent port. This action delays engagement of the clutch and operates contrary to the purpose of the multiple source of pressure, i.e., to reduce the period required to engage the clutch.